Chemical composition for chromium plating



July 1, 19 58 R. C. SMITH CHEMICAL COMPOSITION FOR CHROMIUM PLATING Filed 001;. 28, 1955 REACTOR 4 Sheets-Sheet 1 Hzsl F5 L... HF+ S105 SPRAY DRIER PRODUCT FIG.I

INVENTOR ROBERT C. SMITH BY 673M ATTORNEY July 1, 1958 c, s xg gtg' -l 2,841,540

CHEMICAL COMPOSITION FOR CHROMIUM PLATING Filed Oct. 28, 1955 4 Sheets-Sheet 2 I CATALYZED BATH, |30 F CATHODE CURRENT AMP/SQ FT.

FIG. 2

INVENTOR ROBE RT C. SMITH ATTORNEY July 1, 1958 R. c. SMITH 2,841,540

CHEMICAL COMPOSITION FOR CHROMIUM PLATING Filed Oct. 28, 1955 4 Sheets-$heet3 CATALYZED BATH I30F CATHODE EFFICIENCY PER CENT 6 CATHODE CURRENT, AMP/SQ. FT.

FIG.3

INVENTOR ROBERT C. SMITH BY UFfluw ATTORNEY July 1, 1958 Filed Oct. 28,

COMPRESSIVE 4 Sheets$heet 4 |30F, 275 AMP/SQ. FT.

CATALYZED BATH lO5F, I44 AMP/SQ. FT. CATALYZED BATH THICKNESS OF PLATE, MILS Fl G. 4 INVENTOR ROBERT C. SMITH ATTORNEY United States Patent CHEMICAL COR/ POSITION FOR CHROlVHUM PLATIN G Robert C. Smith, Painesville, Ohio, assignor to Diamond Alkali Company, Cleveland, Ohio, a corporation of Delaware Application October 28, 1955, Serial No. 543,354

11 Claims. (Cl. 2il4--51) This invention relates to improvements in the electro deposition of chromium and more particularly relates to a new and improved catalyst for addition to an aqueous chromium electrolyte and to the preparation and use of such a catalyst in the electro-deposition of chromium.

THE PRIOR ART Although numerous chromium plating baths have here tofore been proposed and used, in many instances chromium remains as one of the most difficult metals to electrodeposit satisfactorily. This difliculty is due, in part at least, to the necessity of reducing chromium from the hexavalent state to the free metal. In an effort to improve the efiiciency of chromium plating, aqueous chromium electrolytes have been proposed and used wherein chromium is presentin a trivalent form. Although chromium can be electrodeposited from a so-called trivalent bath with a lower current consumption than is required in electrodeposition from a hexavalent chromium bath, the quality of the electrodeposits in many cases, up to this time, has not been entirely satisfactory.

Moreover, as in the ease of plating from prior chromium baths, thedeposited chromium frequently has been characterized by a dull appearance and rather poor adherence unless precautions are taken in controlling a number of variables, such as temperature, current density, agitation, and concentration of reagents, within relatively narrow limits which frequently are difficult to maintain in practice,

Up to this time, chromium has generally been deposited from a so-called conventional bath" which consists of chromic acid (CrO and sulfuric acid, these materials being present in amounts to provide a chromic acid: sulfate ratio from about 50:1 to about 150:1. Accordingly, the expression conventional bath, as used herein after, is intended to refer to such an electrolyte and, in most instances, refers. specifically to an aqueous solution consisting of chromic acid and sulfuric acid present in amounts to provide a chromic acid2sulfate ratio of about 100:1.

While conventional chromium plating solutions have long been employed in the electroplating industry and, at times, produce satisfactory electrodeposits, these baths generally have been characterized by relatively poor throwing power, low plating speed and low current efficiency, and have required rather close control to produce bright and adherent chromium electrodeposits.

OBJECTS OF THIS INVENTION It is, therefore, a principal object of this invention to avoid the difficulties heretofore encountered in the electro-deposition of chromium and to provide an improved chromium plating solution.

It is a further object of this invention to provide a new and improved catalyst for an aqueous chromium elec trolyte from which chromium electrodeposits character- 2,841,540 Patented July 1, 1958 ice A further object of the invention is to provide a chromium electrolyte having improved throwing power, covering power, current efiiciency, and other desirable properties.

It is a still further object of this invention to provide a chromium plating solution and catalyst therefor from which chromium electrodeposits of increased hardness may be obtained.

A. still further object of this invention is to provide a chromium plating catalyst which may be added to conventional chromium plating baths to improve both the operating characteristics and the quality of the chromium electrodeposited therefrom.

These and other objects and advantages of the invention will appear more fully from the following description thereof.

Referring now briefly to the accompanying drawings which will be discussed in detail hereinafter:

Fig. 1 schematically illustrates the preparation of a catalyst in accordance with this invention;

Fig. 2 is a graph illustrating the plating rate at varying current densities obtained by the practice of this invention:

Fig. 3 is a graph illustrating the cathode eflicieucies at varying current densities obtained by the practice of the present invention; and

Fig. 4 is a graph demonstrating the stress characteristics of chromium electrodeposited in accordance with the practice of this invention.

THE PRESENT INVENTION It has now been discovered that by employing, as an aqueous electrolyte for chromium electrodeposition, a solution containing, in combination (1) chromium ions, (2) anions of an inorganic acid, and (3) a catalyst of this invention, a chromium electrodeposit characterized by improved corrosion resistance, hardness, and brightness, in addition to other desirable properties, is obtained. Moreover, electrodeposition the foregoing plating solution is characterized by improved throwing power, current efficiency, and covering power over wider temperature and current density ranges than heretofore generally believed practicable.

THE CATALYST OF THIS INVENTION The catalyst of this invention may be added to a conventional chromium electrolyte, at times preferably, although not necessarily, in lieu of a portion of the inorganic acid previously employed, or to another chro mium electrolyte, comprises the product obtained by combining and reacting (1) a chromium compound, preferably a chromium oxide, such as CrO (chromic acid) (2) an organic reducing agent, and (3) at least one substance selected from the group consisting of (a) fluosilicic acid and. (b) the reaction product of hydrogen fluoride,

' preferably an aqueous hydrofluoric acid solution with sirable properties can readily be obtained.

silicon dioxide.

STRUCTURE OF THE CATALYST The precise composition of the catalyst embodying the invention will, of course, vary somewhat, depending on the relative proportions of organic reducing agent, chromic acid and fiuosilicic acid, or hydrogen fluoridesilicon dioxide reaction product, employed, as well as the temperatures, reaction times, and other process conditions utilized. Moreover, the resultant catalyst has not thus far been found subject to characterization as a precise chemical structure but has been established as a reproducible complex material containing silicon, chromium and fluorine. 7

As indicated above, while the catalyst composition will vary somewhat, depending on several factors,

3 catalysts embodying this invention generally may be termed as complex silicon-chromium-fiuorine-containing materials wherein these constituents are present in amounts within the following ranges, the numbers indi cating proportions in terms of percent by weight of the total catalyst:

Chromium 23-28 Silicon 2.5-7.75 Fluorine 24-3 6 Illustrative of specific catalyst compositions in accordance with this invention wherein the quantities are expressed in terms of percent by weight of the total catalyst, are the following:

Catalyst No. Chromium Silicon Fluorine PREPARATION OF A CATALYST OF THE INVENTION Referring now to the accompanying drawing, Fig. 1 illustrates the generally preferred method of producing a catalyst of this invention.

As there shown, suitable amounts of water, chromium trioxide and an organic reducing agent, such as glucose, are introduced into a reactor along with the proper amount of fluosilicic acid or, alternatively, the reaction product of hydrogen fluoride and silicon dioxide. From this reactor there is obtained a catalyst-Water slurry which preferably, as indicated, is spray-dried to remove water and to obtain a finely-divided granular product.

Generally, as indicated, the preparation of a catalyst embodying this invention involves combining (l) chromium trioxide (chromic acid), (2) fluosilicic acid, and (3) an organic reducing agent, portions of the organic reducing agent preferably, although not necessarily, being separately pre-mixed with the chromium trioxide and fluosilicic acid, respectively, these separate materials then being combined to obtain a catalyst-Water mixture which may either be employed as such or, preferably, spray-dried to obtain a dry, granular material.

More specifically, a preferred method of preparing a catalyst of this invention comprises combining chromium trioxide with water and a reducing agent in an amount insufficient completely to reduce all of the hexavalent chromium to the trivalent form, thus forming a first solution. A second solution is then prepared by combining fiuosilicic acid, the commercially available acid, or, if desired, the product prepared by reacting hydrogen fluoride and silicon dioxide, the latter preferably being in a finely-divided state, with suflicient of the same or a different reducing agent to effect complete reduction of the chromium trioxide originally present in the first solution.

This second solution is then combined with the first solution in any suitable reactor, typically glass-lined, provided with agitation and heating means. As indicated above, the thus-formed catalyst-water mixture may, in certain applications, be employed without further treatment. However, in most cases it is desirable to obtain a dry granular product which advantageously can be accomplished by spray-drying the liquid catalyst mixture at an elevated temperature to obtain a finely-divided material. I

Considering the reactants and processing conditions in some detail, attention is directed first to the chromium compound employed. For a number of reasons, chromic acid (CrO is the preferred chromium compound used in preparing a catalyst of this invention. However, other chromium oxides also may be used, e. g., Cr O At times, other chromium compounds which do not introduce undesired ions into a chromium electrolyte also may be used, e. g., chromium carbonate, chromium bicarbonate, and the like.

Glucose, the preferred reducing agent is, of course, but one of several organic reducing agents which can be employed. The expression organic reducing agent," as used in the specification and claims, is intended to include various oxidizable organic substances, e. g., polyhydroxy organic compounds, capable of reducing hexavalent chromium in solution. Suitable organic reducing agents include monoand poly-saccharides, especially the so-called reducing sugars, i. e. monoand/or disaccharides, such as the following:

MONOSACCHARIDES Type Specific Compounds 1. Tetroses (041L104) 2. Pentoses (CEHmOu) 3. Methylpentoses (00111206) 4. Hexoscs (CGHUOO) Other illustrative monosaccharides include Methylhexoses (CIHMOB), Heptoses (C1H14O1), Methylheptoses (C8H10O1), Octoses (03111509), Nonoses (00111509), and Decoses (Owl-120010).

Other types of sugars which may be employed include di-saccharides such as pentose-hexose (C H O methylpentose-hexose (C H O and hexose-hexose (C H O trisaccharides, such as 2 methylpentoseshexoses and 3 hexoses, as well as tetrasaccharides, e. g., 4 hexoses.

Other suitable organic reducing agents include various polyhydroxy compounds, such as glycerine, various alcohols, gelatin, wood flour or sawdust, organic acids, especially monoand/ or dibasic acids such as oxalic, m-aleic, tartaric, acetic, formic, citric, glycollic and succinic acids; esters, such as alkyl formates, acetates, propionates, butyrates, and the like. Those skilled in the art will realize, of course, that various other organic reducing agents also may be employed. The preferred polyhydroxy organic reducing agent in the practice of this invention is glucose, both because of its low cost and its ready availability.

It will be appreciated that the organic reducing agent need not always be of the highest purity. In most, if not all, instances, commercially available compounds of the foregoing types are suitable. At times, even relatively impure materials, such as blackstrap molasses or other form of molasses, tanners sugar, i. e., unrefined corn sugar, bagasse, fruit pulp, and the like, may satisfactorily be used.

The fluosilicic acid (H SiF reactant may comprise a commercially available acid, such as a 30% by weight fluosilicic acid solution, which is preferred. Equivalent amounts of fluosilicic acid may be supplied in other ways as by the alternative method of using the reaction product of hydrogen fluoride and silicon dioxide. When the latter technique is employed, it is desirable to utilize an aqueous hydrogen fluoride solution, such as a 60.4% hydrogen assume fluoride solution, and to react this solution with silicon dioxide in the form of a finely-divided material (preferably minus 325 mesh), silica gel, sand, silica flour or similar materials.

While the proportions of reactants can be varied somewhat, it has been found desirable to employ enough organic reducing agent at least suflicient completely to reduce all of the hexavalent chromium originally present in the chromium-containing reactant employed in preparing the catalyst. The ratio of chromic acid, or other chromium-containing compounds, to fluosilicic acid may vary. However, as a practical matter, it is preferred to employ as small amounts as possible of fluosilicic acid, or the reaction product of hydrogen fluoride and silicon dioxide, with the chromic acid, while still obtaining an economically complete reaction. Generally, a large excess of fiuosilicic acid can be employed without deleteriously affecting the catalyst activity. However, it has been discovered that the bulk density of a spray-dried catalyst varies inversely with the amount of fluosilicic acid employed. Accordingly, insofar as possible, While still maintaining economically complete reaction, it generally is desirable to utilize a minimum fluosilicic acid/chromium oxide ratio in order to achieve a high bulk density.

In order that those skilled in the art may more completely understand the preparation of catalysts of this invention and the methods by which they are prepared, the

following specific examples are offered:

Example 1 Into a 5000 ml. round-bottom flask, equipped with an agitator and. heating means, are introduced 450 ml. of water and 450 gms. of chromium trioxide. To this solution is added 57 gms. of glucose dissolved in 110 ml. of water. The glucose solution is added gradually until the solution boils and then more rapidly in order to main tain the reaction mixture at a boiling temperature.

To a second flask are added 84 gms. of glucose dissolved in 165 ml. of water, and 1800 ml. of 30% by weight fluosilicic acid. This glucose-fluosilicic acid mixture is then added to the chromium-containing solution and external heat applied as necessary to maintain the mixture at boiling temperature for a total of about two hours, until all of the hexavalent chromium originally present is reduced to trivalent chromium.

The thus-obtained reaction product is filtered to remove formed silicon dioxide and is spray-dried at a temperature of approximately 250 F. to obtain 857 gms. of product. The analysis of the product indicates a composition, in terms of percent by weight, of 24.0% chromium, 36.2% fluorine and 5.86% silicon.

Example II Into a 3000 ml. round bottom-flask, equipped with an agitator and heating means, are introduced 500 ml. of water and 250 gms. of chromic acid. To this solution is added 35 gms. of glucose in 59 ml. of water. The glucose solution is added slowly until the reaction mixture is hot and then more rapidly to keep the reaction mixture boiling.

In a separate flask, 65 gms. of glucose in 108 ml. of water is mixed with 438 ml. of a 30% by weight fluosilicic acid solution. The fiuosilicic acid-glucose solution is then added to the chromium-containing solution rapidly enough to keep the reaction mixture at a boil. After the addition is completed, external heat is applied as necessary to maintain the boiling for approximately two hours until the chromium is all reduced to trivalent chromium.

The resultant mixture then is spray-dried at about 250 F. without removal of formed silicon dioxide. There is thus obtained 531 gms. of finely-divided catalyst.

Example III To illustrate the. preparation of a catalyst of this invention using the reaction product of hydrogen fluoride Example IV To illustrate the high bulk density products obtainable in accordance with the preferred practice of this invention, the following data is obtained by repeating the procedure of Example I, using varying ratios of fluosilicic acidzchromium trioxide.

Bulk Density of Product (g. /ml.)

Ml. of 30% by Wgt. Fluosilieii Agid per gram of Chromlc As the foregoing data indicates, the bulk density generally is inversely proportional to the amount of fiuosilicic acid employed. Hence, in most instances, it is desirable to employ a reduced amount of fluosilicic acid, the above indicated 1.85 ml. of 30% fluosilicic acid per gm. of chromic acid being the preferred ratio.

APPLiCATiON OF A CATALYST 0F THiS ii VENTEON IN THE ELECTRODEPUSlTlON OP CHROMIUM Catalysts prepared in accordance with this invention generally may be employed in a variety of chromium electrolytes with advantageous results. However, as those skilled in the art will realize, it often is diificult categorically to specify the precise quantities of catalysts or bath constituents for use in all types of chromium plating. For example, it is well-known that the plating conditions for producing brilliant, decorative plates are not necessarily identical with those desirably employed in forming a hard, non-decorative or so-called engineering plate.

Application of a catalyst of this invention generally may be regulated based upon the fluoride ion concentration, i. e., F- concentration it provides in a chromium electrolyte; this concentration, in combination with a regulated sulfate ion concentration, generally being an accurate and readily determinable index as to the operabiiity and efliciency of an aqueous electrolyte operated in accordance with this invention. The fluoride ion concentration in solution may readily be determined by various means as by the technique disclosed by H. H. Willard and O. B. Winter, in an article appearing in Industrial and Engineering Chemistry, Analytical Edition, No. 5, page '7 (1933).

it will be appreciated that the following preferred concentration ranges, in combination with predetermined sulfate ion concentrations, are not to be construed as limiting the amounts of catalyst which advantageousiy may be employed. in general, amounts less than those be low listed can be employed, although at times some sacrifice in plating efficiency and brightness may be iii-- volved. However, using a concentration less than those set forth below provides improved throwing power and covering power. Similarly, concentrations greater than those set forth below also may bc employed. In such instances, an improved efficienty generally being obtained at the expense of covering power and throwing power.

2,sa1,54.o

As those skilled in the art will realize, in certain applications a desired increase in plating efliciency may well dictate the use of a higher concentration. On the contrary, another application and the necessity of obtaining maximum throwing power may well indicate the use of a lower concentration than those set forth below.

Generally, catalysts of this invention may advantageously be used in amounts to provide a fluorine content in an electrolyte of from about 0.5 to 7.0 gms. per liter. in most instances, the siliconzfluorine ratio of a catalyst should be at least 0.0835:1.0, i. e., at least one part by weight of silicon to each 12 parts by weight of fluorine. Optimum results are obtained when, in addition to these concentrations, the catalyst embodies a chromium to fluorine weight ratio within the range from 0.75:1.0 to 1.0:l.25, a l.0:l.0 ratio being preferred at present.

Preferred catalyst concentrations (in terms of fluoride ion concentration) for decorative plating are within the range from 1.5 to 2.5 gins. per liter, an optimum concentration being between 2.0 to 2.25 gms. per liter. These values reflect the preferred practice and provide optimum results when the sulfate ion concentration in the bath is approximately 1.0 gm. per liter.

In electrodepositiong a hard chromium deposit of the type frequently termed hard chrome or engineering plating, a preferred fluoride ion concentration is within the range from 2.5 to 3.5 gms. per liter, an optimum value being 3.0 gins. per liter. These values indicate the preferred practice and provide optimum results when the sulfate ion concentration is approximately 1.5 gms. per liter.

The concentration of the chromium ion-providing compound, generally CrO in the electrolyte preferably may vary between 150 and 450 gms. per liter, although excellent results are obtained when the chromic acid content is between 100 and 600 gms. per liter. The quantity of inorganic acid employed, such as sulfuric, nitric, or hydrochloric, preferably sulfuric, also may be varied although preferably within the range from about 0.5 gms. per liter to 2.0 gms. per liter.

PLATING CONDITIONS lating solutions embodying the present invention may generally be used to electrodeposit improved coatings of chromium on any conventional cathode material, such as steel, iron, copper, nickel and/or various alloys of these or other metals such as aluminum, magnesium and their alloys. Although a preferred anode material is'a lead-containing material, such as an alloy of lead and tin, other anode materials also may be employed, as are well-known to the art.

To illustrate specific plating conditions and plating bath constituent concentrations, together with temperature and current densities which provide excellent electrodeposits, attention is directed to the following examples, wherein such information is tabulated.

1.0 to 1.41... CIOsIHzSO; IELUO Catalyst 1 concentration Temperature ("11). Current Density (amp/sq. ft.)

165 to 250:1.0 11 to 15 1 24.7% chromium, 31.0% fluorine, and 4.88% silicon.

Example VI To illustrate the excellent results obtained in the quality of chromium electrodeposited and in operating characteristics of a chromium electrolyte embodying invention, a series of plating runs are conducted employing as a chromium electrolyte aqueous. solution consisting of 250 gms. per liter of chromic acid, 1.5 gms. per liter of sulfuric acid, and 10.0 gms. per liter of a catalyst having an analysis of 31.6% by weight fluorine, 24.7% by weight chromium, and 4.88% by weight silicon. The results of such experiments are set forth below:

PART A.PLATING RATE PART B.PLATING EFFICIENCY Reference is made to Fig. 3 which shows the excellent cathode efficiencies varying with current density in electrode-position from the above bath at 130 F.

PA'RT C.HARDNESS OF CHROMIUM ELECTRO- DEPOSITS Knoop hardness values of chromium electrodeposits from above bath are as follows:

Knoop Hardness Thickness Current Number Basis Metal of Cr Plate Density (mils) (amp/sq.

Grams, Grams, 570x 130x 5. 3 310 1,090 b 811 6.1 620 1, 034 h 846 6. 2 900 1,003 b 819 5. 5 310 1, 098 b 850 6. 4 620 l, 094 b 821 6. 4 900 1, 075 b 801 8 300 1,098 b 874 l The steel was hardened to 58-60 Re.

h Hardness measured on polished cross sections. Other measurements were made on the polished surface parallel to the surface of the basis metal.

PART D.-CORRO SION PROTECTION Using the procedure of ASTM Method No. B-117-49T, steel panels coated with 4 mils. ofchromium electrodeposited from the above bath were subjected to salt spray fog at a density of 0.6 ml. per square centimeters per hour at a temperature of 93 to 95 F. for 168 hours. Such panels were completely free from rust except at the edges.

PART E.STRESS OF ELECTRODEPOSITED CHROMIUM Reference is made to Fig. 4 which illustrates that the above bath when operated either at or at F. provides a chromium electrodeposit characterized by compressive stress even at relatively low thicknesses.

Example VII PART A To a 5000 ml. three-necked round-bottom flask equipped with agitation and heating means, are introduced 231.0 gms. of 60.4% hydrofluoric acid solution and 71.0 gms. of silicon dioxide (Ottawa sand flour). This mixture is allowed to stand overnight. To the resultant mixture is then added 500 ml. of water, 250 gms. of chromic acid and 37 gms. of glucose in 67 ml. of water. To the thusformed solution there is then gradually added, at a rate sufficient to maintain the reaction mixture at a boil, 55.5 gms. of glucose in 100 ml. of solution. The thus-obtained solution is spray dried using a spray drier maintained at 450 F. having an upper wall temperature of 250 F an outlet temperature of 210 F., at a feed rate of 100 ml. per minute.

PART B 7 Using the catalyst spray dried in accordance with part A, three chromium plating solutions are prepared. These solutions, hereinafter termed Nos. 1, 2, and 3 respectively, have the following composition:

No.1, No. 2, No.3, gms. per gms. per gins. per liter liter liter Chrornlc Acid 250 250 250 Sulfuric Acid... 1 1 1 Catalyst 4. 2 6. 3 8. 4

The above baths are prepared and allowed to stand for two hours before electrolysis is begun. Using such baths, electrodeposition of chromium therefrom is carried out at a temperature of 130 F. and a current density of 300 amps per square foot. Using a conventional Hull Cell testing procedure, the following results are obtained:

To illustrate the efliciency of baths embodying the present invention, bronze rods are polished and cleaned and electroplated with chromium from the above bath No. 3, a copper coulometer being used in series With the bath.

Using the bath at 130 F. with a current of 5 amperes, for a period of 60 minutes in electrodeposition, a chromium deposit weighing 0.3590 gm. was obtained. This weight reflects a plating efficiency of 20.9%.

It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited, since changes and alterations therein may be made which are within the full intended scope of this inveniton as defined by the appended claims.

What is claimed is:

1. The reaction product obtained by chemically reacting a chromium compound, an organic reducing agent and fiuosilicic acid in amounts sufficient to yield a material containing about 23 to 28% chromium, 2.5 to 7.75% silicon and 24-36% fluorine.

2. The reaction product according to claim 1 wherein the chemical reaction is carried out using sufiicient reducing agent to effect the reduction of all chromium to the trivalent form.

3. A reaction product according to claim 1 which is obtained by combining a hexavalent chromium compound with water and a reducing agent in an amount insufficient completely to reduce all of the hexavalent chromium to the trivalent form, thus forming a first solution and adding thereto fluosilicic acid and a reducing agent in an amount sufiicient to efiect complete reduction of the chromium to trivalent form.

4. The reaction product according to claim 3 wherein the material is spray dried to obtain a finely-divided material.

5. A reaction product comprising 23-28% chromium, 2.5 to 7.75% silicon, and 24-36% fluorine obtained by chemically reacting a hexavalent chromium compound, fluosilicic acid and an organic reducing agent at an elevated temperature until all of the hexavalent chromium originally present is completely reduced to trivalent chromium.

6. The reaction product according to claim 5 wherein the material is spray dried to obtain a finely-divided product.

7. In the process of electrodepositing chromium from an aqueous electrolyte containing chromium ions, and ions of an inorganic acid, the improvement which includes incorporating into said electrolyte an amount of the product defined by claim 1 such that the fluorine content of said electrolyte is from about 0.5 to 7.0 grams per liter.

8. In the process of claim 7 the improvement which includes incorporating in said electrolyte the: product defined by claim 2.

9. In the process of claim 7 the improvement which includes incorporating in said electrolyte the product defined by claim 3.

10. In the process of claim 7, the improvement which includes incorporating in said electrolyte the product defined by claim 4.

11. In the process of electrodepositing chromium from an aqueous electrolyte containing chromium ions, and ions of an inorganic acid, the improvement which includes incorporating into said electrolyte an amount of the product defined by claim 5 such that the fiourine content of said electrolyte is from about 0.5 to 7.0 grams per liter.

References Cited in the file of this patent UNITED STATES PATENTS 1,815,081 Sohn et al. July 21, 1931 1,844,751 Fink et al. Feb. 9, 1932 1,928,284 Fink et a1. Sept. 26, 1933 2,640,021 Passal May 26, 1953 FOREIGN PATENTS 617,292 Great Britain Feb. 3, 1949 OTHER REFERENCES Gilman: Inorganic Reactions, 1930, pages 107 and 222.

Talipov et al.: Chemical Abstracts, vol. 48 (February 1954),pp. 1869-1870.

Mellor: Comprehensive Treatise on Inorganic Chem., vol. 6, page 956. 

1. THE REACTION PRODUCT OBTAINED BY CHEMICALLY REACTING A CHROMIUM COMPOUND, AN ORGANIC REDUCING AGENT AND FLUOSILICIC ACID IN AMOUNTS SUFFICIENT TO YIELD A MATERIAL CONTAINING ABOUT 23 TO 28% CHROMIUM, 2.5 TO 7.75% SILICON AND 24-36% FLUORINE.
 7. IN THE PROCESS OF ELECTRODEPOSITING CHROMIUM FROM AN AQUEOUS ELECTROLYTE CONTAINING CHROMIUM IONS, AND IONS OF AN INORGANIC ACID, THE IMPROVEMENT WHICH INCLUDES INCORPORATING INTO SAID ELECTROLYTE AN AMOUNT OF THE PRODUCT DEFINED BY CLAIM 1 SUCH THAT THE FLUORINE CONTENT OF SAID ELECTROLYTE IS FROM ABOUT 0.5 TO 7.0 GRAMS PER LITER. 